IMMUNITY 57 



B. Passive Immunity, i.e. produced in one animal by injection 

 of the serum of another animal highly immunized by the 

 methods of A. 



1. By antitoxic serum, i.e. the serum of an animal highly 



immunized against a particular toxin. 



2. By antibacterial serum, i.e. the serum of an animal highly 



immunized against a particular bacterium in the living 

 and virulent condition. 



The Mechanism of Immunity. — The study of the 

 mechanism of immunity, that is, the process which 

 occurs within the body as a defense against the in- 

 vading bacteria, has been enormous and presents 

 problems too abstruse and complex for the under- 

 standing of any but the scientists themselves. As a 

 result of these researches, which involve such industry, 

 application, patience, and profound learning that we 

 stand amazed at their depth, there has grown to be 

 more or less well-defined theories of the mechanism 

 of immunity, although the most learned among the 

 investigators hesitates the least to admit the hmits 

 of this knowledge. 



Antitoxins. — In 1890 Behring and Kitasato dis- 

 covered that if the. blood-serum of an immunized 

 animal was combined with an appropriate amount 

 of toxin (in this case tetanus), and the com- 

 bination injected into a susceptible animal, there 

 was neither death nor infection; In other words, 



